1. Field of the Invention
The present invention relates generally to high temperature sliding elements for use in sealing and bearing elements, and more specifically to ceramic high temperature sliding elements and a method for preventing the same from wear at high temperatures, wherein high temperature sliding characteristics can be improved by modifying the surface of ceramics without deteriorating the intrinsic features of ceramics having an excellent heat resistance.
2. Description of the Prior Art
As sliding elements used in a high temperature atmosphere such as sealing and bearing elements, conventionally, there have been employed heat resisting metallic materials of cobalt (Co) or nickel (Ni) base alloys. At present, however, the upper allowable temperature limit is about 800.degree. C. from the standpoint of heat resistance related to oxidation resistance, high temperature strength or the like.
Accordingly, there have been attempted the use of ceramic materials, because of its excellent physical, chemical and mechanical properties in a high temperature atmosphere as compared with metals. However, the ceramics which are generally believed to be antiwear material are practically not excellent in wear resistance, rather involving such drawbacks that wear increases markedly with increasing temperature. For the reason as described above, ceramic materials having satisfactory high temperature characteristics when used as structure materials are not available as high temperature sliding elements when used as they are.
In the case where used as sliding elements in a high temperature atmosphere, ceramic materials suffer wear at local high contact pressure portions and produce wear debris. Due to abrasive wear by the wear debris and an increase of the surface roughness, wear is accelerated in both the sliding element and the counterpart. In the case where the counterpart is made of a material such as a metal softer than ceramics, since the produced wear debris is embedded or accommodated on the surface of the counterpart, the abrasive wear by wear debris can be suppressed to some extent. However, in the case where the counterpart is also made of ceramics, since the wear debris is not readily embedded or accommodated, wear increases markedly, thus causing change in dimensions to such an extent that cannot be practically acceptable.
To solve the above problem, so far various methods have been developed, and some sliding elements which can improve the antiwear properties have been proposed. One of them is a silicon nitride material obtained by adding boron nitride (BN) to ceramic material as solid lubricant and by sintering the surface layer thereof (Japanese Patent Kokai Publication No. 59-137375). In this ceramic material, it is true that the wear characteristics can be improved to some extent. However, there still exists another problem in that high temperature characteristics, in particular high temperature strength, which should be inherently satisfied by the ceramic material are lowered.